Polishing method and polisher used in the method

ABSTRACT

According to the present invention, there is provided a polishing method having the steps of forming a film to be polished, having a depressed portion and a protruding portion on a surface of a substrate, and polishing the film to be polished by relatively moving the substrate and a polishing table, while pressing the substrate having the film to be polished, onto a polishing cloth of the polishing table and supplying a polishing solution containing polishing grains, between the film to be polished and the polishing cloth, wherein an organic compound having a molecular weight of 100 or more, and containing at least one hydrophilic group selected from the group consisting of COOM 1  (M represents an atom or a functional group which can form a salt when substituted with a hydrogen atom of a carboxyl group), SO 3 H (sulfo group) and SO 3 M 2  (M 2  represents an atom or a functional group which can form a salt when substituted with a hydrogen atom of a carboxyl group) is added to the polishing solution. Further, there are provided a polishing solution in which polishing grains are dispersed into a dispersion medium, and a polishing agent containing an organic compound having a molecular weight of 100 or more and containing at least one hydrophilic group selected from the group consisting of COOM 1  (M represents an atom or a functional group which can form a salt when substituted with a hydrogen atom of a carboxyl group), SO 3 H (sulfo group) and SO 3 M 2  (M 2  represents an atom or a functional group which can form a salt when substituted with a hydrogen atom of a carboxyl group) added to the polishing solution.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a polishing method and apolishing agent used in a semiconductor element manufacturing method,more specifically to a polishing method and a polishing agent used in astep for smoothing the surface of a substrate, especially, smoothing aninterlayer insulation film, a step of forming a buried metal wiringportion, a step of forming a buried element separation film, or a stepof forming a buried capacitor, or the like.

[0003] 2. Description of the Related Art

[0004] With regard to current ultra-super-large-scale integratedcircuits, there is a trend towards reducing the sizes of a transistorand other semiconductor elements and increasing the mounting density.Accordingly, various micro processing techniques are being studied anddeveloped, and the design rule is already set to an order of a sub-halfmicron.

[0005] One of the techniques being developed in order to satisfy thestrict requirement of the micro processing is a CMP (chemical mechanicalpolishing) technique. This technique is essential to the manufacturingprocess of a semiconductor, in particular, when smoothing an interlayerinsulation film, forming a plug, forming a buried metal wiring portion,separation of a buried element, forming a buried capacitor and the like.

[0006]FIGS. 1A to 1E are cross sections of an interlayer insulationfilm, illustrating a step of smoothing the film by use of a CMPtechnique. First, as shown in FIG. 1A, a silicon oxide film 2 is formedon a silicon substrate 1 a ratio of whose protruding portions occupies50% of the entire surface, and a first Al wiring portion 3 having awidth of 0.3 μm and a height of 0.4 μm is formed on the silicon oxidefilm 2 by a general lithography method and a general etching method.Next, as shown in FIG. 1B, a silicon oxide film 4 having a thickness of1.3 μm is formed by a plasma CVD method, and then an abrasion process iscarried out so as to smooth the silicon oxide film 4. Various changes incross-sectional shape of the film are illustrated in FIGS. 1C to FIG.1E. FIG. 1C shows a cross sectional shape of the film in the case wherethe abrasion process is completed at an ideal position, whereas FIGS. 1Aand 1E each show a cross sectional shape in the case where the abrasionprocess is excessively carried out.

[0007] With the conventional abrasion technique, the abrasion ratechanges along with an elapse of time, and therefore it is very difficultto stop the processing at an ideal position as shown in FIG. 1C.Further, in the case where there is a wide space between Al wiringportions, that is, when the silicon oxide film 4 is wide, the centerportion of the silicon oxide film has priority to other portions inabrasion, thus causing a so-called dishing.

[0008] In the case where the abrasion is excessively carried out asshown in FIG. 1D, the pressure resistance between a second Al wiringportion (not shown) formed on the silicon oxide film 4 and the first Alwiring portion 3 is deteriorated. Further, the abrasion is excessivelycarried out as shown in FIG. 1E, the first Al wiring portion 3 is insome cases disconnected.

[0009] In order to solve the above-described drawbacks, it has beenproposed a technique in which an anti-abrasion film 5 made of a materialsuch as Si₃N₄, having an abrasion rate lower than that of ato-be-polished member (in this case, silicon oxide film) is formed onthe wide silicon oxide film 4 as shown in FIG. 2 (Jap. Pat. Appln. KOKAIPublication No. 5-315308). However, this technique entails a problem inwhich the selection rate (the abrasion rate of the Si₃N₄ film/theabrasion rate of the silicon oxide film) cannot be set high, thusincreasing the number of steps of forming, removing and the like of theanti-abrasion film 5.

SUMMARY OF THE INVENTION

[0010] The present invention has been proposed in consideration of theabove problems, and the object thereof is to provide an abrasion methodof polishing a desired protruded portion at high efficiency withoutcausing dishing.

[0011] According to the first aspect of the present invention, there isprovided a polishing method comprising the steps of: forming a film tobe polished, having a depressed portion and a protruding portion on asurface of a substrate; and polishing the film to be polished byrelatively moving the substrate and a polishing table, while pressingthe substrate having the film to be polished, onto a polishing cloth ofthe polishing table and supplying a polishing solution containingpolishing grains, between the film to be polished and the polishingcloth; wherein an average distance between a surface of the depressedportion of the film to be polished and a surface of the polishing clothduring polishing is set larger than an average diameter of the polishinggrains.

[0012] According to the second aspect of the present invention, there isprovided a polishing method comprising the steps of: forming a film tobe polished, having a depressed portion and a protruding portion on asurface of a substrate; and polishing the film to be polished byrelatively moving the substrate and a polishing table, while pressingthe substrate having the film to be polished, onto a polishing cloth ofthe polishing table and supplying a polishing solution containingpolishing grains, between the film to be polished and the polishingcloth; wherein a frictional coefficient between the film to be polishedand the polishing solution during polishing is larger than a frictionalcoefficient between the polishing cloth and the polishing solution.

[0013] According to the third aspect of the present invention, there isprovided a polishing method having the steps of: forming a film to bepolished, having a depressed portion and a protruding portion on asurface of a substrate; and polishing the film to be polished byrelatively moving the substrate and a polishing table, while pressingthe substrate having the film to be polished, onto a polishing cloth ofthe polishing table and supplying a polishing solution containingpolishing grains, between the film to be polished and the polishingcloth; wherein an organic compound having a molecular weight of 100 ormore, and containing at least one hydrophilic group selected from thegroup consisting of COOM₁ (M represents an atom or a functional groupwhich can form a salt when substituted with a hydrogen atom of acarboxyl group), SO₃H (sulfo group) and SO₃M₂ (M₂ represents an atom ora functional group which can form a salt when substituted with ahydrogen atom of a carboxyl group) is added to the polishing solution.

[0014] According to the fourth aspect of the present invention, there isprovided a polishing method comprising the steps of: forming a film tobe polished, having a depressed portion and a protruding portion on asurface of a substrate; and polishing the film to be polished byrelatively moving the substrate and a polishing table, while pressingthe substrate having the film to be polished, onto a polishing cloth ofthe polishing table and supplying a polishing solution containingpolishing grains, between the film to be polished and the polishingcloth; wherein a content of an organic compound in of the polishingsolution supplied is varied during polishing.

[0015] Further, there are provided a polishing solution in whichpolishing grains are dispersed into a dispersion medium, and a polishingagent containing an organic compound having a molecular weight of 100 ormore and containing at least one hydrophilic group selected from thegroup consisting of COOM₁ (M represents an atom or a functional groupwhich can form a salt when substituted with a hydrogen atom of acarboxyl group), SO₃H (sulfo group) and SO₃M₂ (M₂ represents an atom ora functional group which can form a salt when substituted with ahydrogen atom of a carboxyl group) added to the polishing solution.

[0016] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention and, together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0018]FIGS. 1A to 1E are cross sections of a film, illustrating aconventional polishing method;

[0019]FIG. 2 is a cross section of another film, illustrating anotherconventional polishing method;

[0020]FIGS. 3A and 3B are cross sections of a film, illustrating a filmto be polished by the polishing method of the present invention;

[0021]FIG. 4 is a diagram briefly showing a polishing device used in thepolishing method of the present invention;

[0022]FIGS. 5A to 5C are cross sections showing a change in shape of asample polished by the polishing method of the present invention;

[0023]FIG. 6 is a graph illustrating a change in remaining thicknessesof a protruding portion and a depressed portion, created by polishingalong with an elapse of time;

[0024]FIG. 7 is a graph showing the relationship between a polishingrate of each of the protruding and depressed portions, and the contentof a polycarboxylic ammonium salt in the polishing solution;

[0025]FIG. 8 is graph showing the relationship between an intrafacialuniformity and an intrafacial maximum step of the film after polishing,and the content of a polycarboxylic ammonium salt in the polishingsolution;

[0026]FIG. 9 is a graph showing the relationship between a polishingrate of a depressed portions, and the viscosity of a polishing solution;

[0027]FIG. 10 is a graph showing the relationship between the additionof polycarboxylic ammonium salt and the viscosity of a polishingsolution;

[0028]FIG. 11A is a diagram illustrating the mechanism of the polishingmethod of the present invention;

[0029]FIG. 11B is a diagram illustrating the conventional polishingmethod;

[0030]FIG. 12 is a diagram showing a film to be polished by thepolishing method of the present invention;

[0031]FIG. 13 is a graph showing the relationship between the polishingrate and the content of a polycarboxylic ammonium salt in the polishingsolution in the case where the diameter of polishing grains is varied;

[0032]FIG. 14 is a graph showing the relationship between the polishingrate and the content of a polycarboxylic ammonium salt in the polishingsolution in the case where the polishing pressure is varied;

[0033]FIGS. 15A to 15D are cross sections of a film in the case wherethe polishing method of the present invention is applied to theformation of a buried metal wiring portion;

[0034]FIGS. 16A to 16D are cross sections of a film in the case wherethe polishing method of the present invention is applied to theformation of a contact portion; and

[0035]FIG. 17 is a graph indicating the motor current value of apolishing table with respect to a change in the content of apolycarboxylic ammonium salt in the polishing solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The inventors of the present invention has achieved the presentinvention based on the following discovery. That is, when an organiccompound having a very strong interaction with a to-be-polished film isadded, the average distance between the surface of a depressed portionof the to-be-polished film and the surface of the polishing cloth duringa polishing process is rendered wider than the average diameter ofpolishing grains, or the friction coefficient between the to-be-polishedfilm and the polishing solution during a polishing process is renderedlarger than the friction coefficient between the polishing cloth and thepolishing solution. Therefore, only the protruding portion of theto-be-polished film having an irregular surface, is polished withpriority, and when the surface is flattened, the polishing rate islowered.

[0037] In the polishing method of the present invention, an organiccompound having a strong interaction with a film to be polished, forexample, an organic compound containing a hydrophilic group such as acarboxyl group or a sulfo group and having a molecular weight of 100 ormore, is added to a polishing solution, thus forming a stagnant? layeron the surface of the depressed portion of the to-be-polished film.Consequently, the average distance between the surface of the depressedportion of the film and the surface of the polishing cloth in apolishing process is rendered wider than the average diameter of thepolishing grains.

[0038] In this polishing process, the number of polishing grainseffectively contributing to the polishing of the depressed portion ofthe film is apparently reduced, and therefore the protruding portion ofthe film is polished with priority. Further, the polishing rate islowered as the surface of the film is flattened. Consequently, thepolishing can be stopped at the end point of polishing while preventingthe occurrence of dishing. Therefore, the thickness of the polishingfilm can be controlled, and only the protruding portions can be removedso as to flatten the surface of the film.

[0039] In the polishing method of the present invention, the polishingcan be controlled also by making the frictional coefficient between afilm to be polished and a polishing solution in a polishing process,larger than the frictional force between a polishing cloth and thepolishing solution. Therefore, the surface of the film to be polishedcan be flattened at high accuracy.

[0040] Further, in the polishing method of the present invention, theorganic compound having a strong interaction with the to-be-polishedfilm is added and, the polishing rate can be varied during the polishingprocess, thus improving the controllability of the polishing. Forexample, with the addition of the organic compound to the polishingsolution in the middle of the polishing process, the polishing rate canbe increased until the middle of the polishing process, and the surfacesmoothing degree of the film to be polished can be improved at the finalstage of the polishing process.

[0041] In the first to fourth aspects of the invention, silicon, quartz,sapphire, Al₂O₃, and a compound of an element of Group III and anelement of Group V of the periodic table, are used as materials for eachsubstrate. As a film to be polished, a film mainly containing SiO₂,α-Si, poly-Si, SiON, SiOF, BPSG (boro-phospho-silicate glass), PSG(phospho-silicate glass), SiN, Si₃N₄, Si, Al, W, Ag, Cu, Ti, TiN, Au, Ptor the like can be used.

[0042] As the polishing powder, the powder mainly containing SiO₂, CeO₂,Al₂O₃, Fe₂O₃, SiC, SiN, ZrO₂, TiO₂, C (diamond) or the like, can beused. A polishing solution can be prepared by dispersing the polishingpowder into a dispersion medium such as pure water or alcohol. It ispreferable that the average grain diameter of the polishing powdershould be 0.01 to 5.0 μm. This is because, if the average grain diameterof the powder is less than 0.01 μm, the polishing rate is excessivelylowered, whereas if it exceeds 5.0 μm, the surface of the film may bedamaged.

[0043] A generally used polishing cloth can be used as the polishingcloth. The pressing force, the supplying amount and the relativerotation speed between the substrate and the polishing table, used forthe polishing process can be set under general conditions.

[0044] In the first aspect, the fact that the average distance betweenthe surface of the depressed portion of the to-be-polished film and thesurface of the polishing cloth in the polishing process becomes largerthan the average diameter of the polishing grains, means the state inwhich the polishing rate for polishing a depressed portion or a flatportion of the film is proportional to the content of an organiccompound. More specifically, in FIG. 7, the crosspoint made by a tangentline extending from the side representing that polycarboxylic ammoniumsalt is not added and another line extending from the side wherepolycarboxylic ammonium salt is added in great amount, that is, theadding amount of polycarboxylic ammonium salt, falls within a rangebetween 2.2 weight % and 4.5 weight %. The reason why the polishing rateonly gradually changes is that the grain diameter is not unified.

[0045] In the second aspect, the fact that the frictional coefficientbetween the surface of the depressed portion of the to-be-polished filmand the polishing solution in the polishing process becomes larger thanthe frictional coefficient between the polishing cloth and the polishingsolution, means the state in which the torque applied to the polishingtable having the polishing cloth is rendered constant.

[0046] In the third aspect, the organic compound to be added to thepolishing solution should preferably be a high-molecular polycarboxylicammonium salt or a high-molecular ammonium polysulfonate. Examples ofthe hydrophilic group contained in these organic compound are —COOH(carboxyl group), —COOM (M: an atom or a functional group which can forma salt when substituted with the hydrogen atom of the carboxyl group,for example, —Na, —NH₄), —SO₃H (sulfo group) and —COOM (M: an atom whichcan form a salt when substituted with the hydrogen atom of the sulfogroup, for example, —Na, —NH₄). Of these hydrophilic groups, —COOM,—SO₃H and —SO₃M are preferable since they are easily dissolved to water.Further, the molecular weight of each organic compound should preferablybe 100 or more. This is because, if the molecular weight of the organiccompound is less than 100, a stagnant layer, which will be explainedlater, cannot be easily formed. A specific example of the organiccompound to be added to the polishing solution is polycarboxylicammonium salt, polysulfonic ammonium salt, or the like.

[0047] In the fourth aspect, as the method of adjusting the content ofan organic compound in the polishing solution to be supplied, atechnique in which two or more polishing solution supplying tankscontaining polishing solutions having different content of the organiccompounds and the supply of a solution from each tank is switched over,or a technique in which the supplying amount from each of the polishingsolution supplying tank and the organic compound supplying tank isadjusted, can be used.

[0048] In the first to fourth aspects of the present invention, the loadapplied to the film to be polished, that is, a pressure for pressing thesubstrate on the polishing table (pressing force), the diameter of thepolishing grains contained in the polishing solution, the content of theorganic compound in the polishing solution in which polishing grains aredispersed and the rotation number of the substrate with respect to thepolishing table are appropriately set, so as to satisfy the requirementsof the first to fourth aspects. The rotation number of the substrate, inthe case where a vacuum chuck holder or the like is used for supportingthe substrate, can be adjusted by adjusting the rotation number of theholder.

[0049] Examples of the present invention will now be described withreference to accompanying drawings.

EXAMPLE 1

[0050] As shown in FIG. 3A, a silicon oxide film 12 was formed as aninsulation film on a silicon substrate 11 a ratio of protruding portionsof which was 50% of the entire surface of the substrate, and an aluminumwiring portion 13 having a width of 0.3 μm and a height of 0.4 μm wasformed on the silicon oxide film 12 by a general photolithography methodand a general etching method. Next, as shown in FIG. 3B, a silicon oxidefilm 14 having a thickness of 1.3 μm was formed by a plasma CVD method,thus forming a sample 20. In this figure, reference numeral 15 denotes aprotruding portion and numeral 16 denotes a depressed portion.

[0051] Next, the sample 20 was subjected to a CMP by use of a polishingapparatus shown in FIG. 4, thus smoothing the interlayer insulationfilm. The apparatus consists of a rotatable polishing table 21, apolishing cloth adhered onto the polishing table 21, a rotatable vacuumchuck holder 23 located above the polishing table 21 and a polishingsolution supplying pipe 24 connected to a polishing solution tank andhaving an ejecting portion extending close to the cloth 22. The sample20 was vacuum-chucked by the vacuum chuck holder 23 so that the surfaceto be polished faced the polishing cross 22. The polishing solutionsupplying pipe 24 includes means for controlling the supplying amount ofthe polishing solution. It should be noted that the polishing cloth 22was a resin-impregnated unwoven cloth having a thickness of 1.2 mm and ahardness of 85.

[0052] In the CMP, as the polishing solution, there was used a typeobtained by dispersing oxide cerium grains having an average diameter of0.6 μm to pure water at a ratio of 1.0 weight %, adding 2.5 weight % ofpolycarboxylic ammonium salt thereto, and adjusting the viscosity of thesolution to 3.0 cP while measuring it with an Ostwald's viscometer. Thepolishing conditions were set as follows. That is, the polishingpressure was set to 300 gf/cm² and the rotation number of the polishingtable and the vacuum chuck holder was set to 100 rpm (these conditionswere called as the standard conditions for polishing). It should benoted that the pressure of the sample 20 abutting to the polishing cloth22 could be controlled arbitrarily by compressed air.

[0053] A change in the cross sectional shape along with an elapse oftime in the case where the sample 20 was polished by the polishingmethod of the present invention illustrated in FIGS. 5A to 5C. FIG. 5Ashows a cross sectional shape after 60 seconds of the polishing process,and FIG. 5B shows a cross sectional shape after 120 seconds of thepolishing process. After the protruding portions were polished and aflat surface was obtained as shown in FIG. 5B, the polished did notsubstantially progress, thus always obtaining an ideal cross sectionalshape as shown in FIG. 5C. As can be seen from FIG. 5C, the dishing canbe significantly controlled.

[0054]FIG. 6 shows a change in remaining thickness of the protrudingportion and the depressed portion, along with an elapse of time. FIG. 6also shows the result of a CMP carried out under the standard conditionsby use of a polishing agent and prepared by dispersing oxide ceriumgrains having an average grain diameter of 0.6 μm into pure water at aratio of 1.0 weight %. As can be seen from FIG. 6, in the polishingmethod of the present invention, when the difference in thicknessbetween the protruding portion 15 and the depressed portion becomessmall (circled portion in the figure), both the protruding portion 15and the depressed portion 16 are not polished any further. Thus, thepolishing process can be stopped at the point where an ideal remainingthickness is obtained, and the dishing can be prevented. It isconsidered that such an effect can be obtained for the following reason.That is, with the addition of polycarboxylic ammonium salt to thepolishing solution, the average distance between the main surface of thesilicon oxide film and the main surface of the polishing cross isrendered larger than the average diameter of the cerium oxide grains, orthe frictional coefficient between the silicon oxide film and thepolishing solution in the polishing process is rendered lower than thefrictional coefficient between the polishing cloth and the solution.More specifically, when the average distance between the main surface ofthe silicon oxide film and the main surface of the polishing cloth iswidened, the cerium oxide grains do not easily act on the surface ofsilicon oxide. Consequently, the polishing rate is gradually lowered ininversely proportional to the viscosity of the polishing liquid. Whenthe distance is widened to have a certain width or more, the polishingrate is made constant. When the polishing is carried out in theabove-described state, the cerium oxide grains act the protrudingportion with priority. However, after the irregularity of the surface isreduced, the grains no longer act on the protruding portion, thusachieving a good controllability of the film thickness and suppressingthe dishing.

[0055] In contrast, with the conventional polishing method, even afterthe difference in film thickness between the protruding portion 15 andthe depressed portion 16 would be reduced, both the protruding portion15 and the depressed portion 16 are polished.

[0056] Next, in order to examine the effect of polycarboxylic ammoniumsalt added to the polishing solution, the sample 20 shown in FIG. 3B issubjected to the CMP process under the standard conditions by use of apolishing solution, which was prepared by adjusting the amount ofaddition of polycarboxylic ammonium salt. The results of thisexamination are shown in FIGS. 7 and 8.

[0057]FIG. 8 is a graph showing the relationship between the polishingrate of each of the protruding portion 15 and the depressed portion 16,and the content of a polycarboxylic ammonium salt in the polishingsolution. As can be understood from FIG. 7, as the amount of addition ofpolycarboxylic ammonium salt is increased, the polishing rate isreduced. In specific, the polishing rate for the depressed portion 16 ismade constant at a point of 2.2 weight % or higher, whereas thepolishing rate for the protruding portion 15 is abruptly decreased at apoint close to 1.0 weight %, and is made constant at a point of 5.0weight % or higher.

[0058]FIG. 8 is a graph showing the relationships between the content ofa polycarboxylic ammonium salt in the polishing solution, and both theintrafacial uniformity and the intrafacial irregularity of the filmafter the CMP process. As can be seen from FIG. 8, when the content isin a range of 0.4 to 4.5 weight %, an ideal cross sectional shape with ahigh intrafacial uniformity and a less intrafacial irregularity, can beobtained.

[0059] Therefore, as the polycarboxylic ammonium salt is added to thepolishing solution, the selection ratio in which the protruding portion15 and the depressed portion 16 are polished, is raised. Consequently,only the protruding portion 15 can be polished with priority, and theintrafacial uniformity is improved when the content is 0.4 to 4.5 weight%. Further, in a range of the content of 2.2 to 4.5 weight %, when thesurface is smoothed, the polishing no longer proceeds. Thus, thepolishing can be stopped at the final point of the polishing with a highcontrollability.

[0060] Next, in order to examine, in more detail, the effect ofpolycarboxylic ammonium salt added to the polishing solution, the sample20 shown in FIG. 3B is subjected to the CMP process under the standardconditions by use of polishing solutions each having a differentviscosity which was prepared by adding polycarboxylic ammonium salt orethanol or polyvinyl alcohol having a molecular weight of 20000 to30000. The results of this examination are shown in FIG. 9. FIG. 9 is agraph showing the relationship between the polishing rate of thedepressed portion 16, and the viscosity of the polishing solution. Ascan be understood from FIG. 9, in the case of a solution to whichpolycarboxylic ammonium salt is added, the proportion in decreasing ofpolishing rate is made constant at a point close to 2.0 cP. From thecharacteristics of the polishing solution to which polycarboxylicammonium salt was added, show in FIG. 9, it can be concluded that onlythe protruding portion 15 was be polished with priority, and when thesurface is smoothed, the polishing no longer proceeds. In the case wherea polishing solution to which ethanol or polyvinylalcohol was added, wasused, as the viscosity is increased, the polishing rate is reduced.However, the proportion in decreasing of polishing rate is not madeconstant.

[0061] Next, the relationship between the addition of polycarboxylicammonium salt and the viscosity of the polishing solution was examinedby use of a rotation viscometer and an Ostwald's viscometer, and theresults thereof are shown in FIG. 10. As can be understood from FIG. 10,as the amount of polycarboxylic ammonium salt is increased, theviscosity of the polishing solution is linearly increased. Therefore, itcan be concluded that the polishing solution to which polycarboxylicammonium salt was added, was a Newton viscous fluid. It should be notedthat the value measured by the rotation viscometer was smaller than thatof the Ostwald's viscometer. This is because the apparent viscosity isincreased since the interaction between the polishing agent and glass ishigh.

[0062] Next, the operation of the polishing method of the presentinvention, which is assumed from the above results, will now bedescribed.

[0063]FIG. 11A is a cross sectional view illustrating the case where thesample 20 shown in FIG. 3B is subjected to an CMP process by thepolishing method of the present invention. As can be understood fromFIG. 11A, when the polishing solution to which a predetermined amount ofpolycarboxylic ammonium salt was added, is used in the CMO process, thedistance between the main surface of the silicon oxide film 14 and thepolishing cloth 22 is widened due to the viscosity of the polishingsolution. At the same time, the protruding portion 15 is polished by thepolishing grains 31. In the depressed portion 16, the distance betweenthe main surface of the silicon oxide film 14 and the polishing cloth 22becomes larger than the average grain diameter of the polishing grains31, and a stagnant layer 33 made of polishing solution 32 containingpolishing grains 31 is formed. Consequently, the apparent number of thepolishing grains acting effectively in the-polishing of the depressedportion 16 is reduced. Thus, the protruding portion 15 is polished withpriority, and when the surface of the silicon oxide film 14 isflattened, the further polishing is not carried out.

[0064] On the other hand, FIG. 11B is a cross section illustrating thecase where the sample 20 shown in FIG. 3B was subjected to the CMPprocess by use of a polishing solution to which ethanol orpolyvinylalcohol was added.

[0065] As can be understood from FIG. 11B, the distance between the mainsurface of the silicon oxide film 14 and the polishing cloth 22 iswidened due to the viscosity of the polishing solution; however nostagnant layer is formed in the depressed portion 16. Therefore, thepolishing grains 31 act equally on the protruding portion 15 and thedepressed portion 16. For this reason, it becomes difficult to take aselection ratio between the depressed portion and the depressed portion16. In this case, it is a possibility that the viscosity of thepolishing solution is further increased in order to reduce the number ofpolishing grains acting on the depressed portion 16. However, such anidea is not preferable since an increase in viscosity also lowers thepolishing rate of the protruding portion 15.

[0066] Next, the phenomenon in which the polishing no longer proceedsafter the surface is smoothed, will now be discussed in further detail.In connection with this phenomenon, a change in the viscosity of thepolishing solution, which occurs when the grain diameter of polishinggrains used or the polishing pressure are varied, was examined. Asample, which was prepared by forming a silicon oxide film 42 having athickness of 1 μm on a silicon substrate 41 by a plasma CVD method, asshown in FIG. 12, that is, a type having a smooth surface, was used.

[0067] First, the relationship between the content of the polycarboxylicammonium salt in the polishing solution and the polishing rate alongwith variable diameter of the polishing grains was examined, and theresults thereof are shown in FIG. 13. The polishing solution, which wasobtained by dispersing cerium oxide grains having an average graindiameter of 2.0 μm into pure water at a ratio of 1.0 weight %, was used.The polishing conditions were the same as the standard conditions, andthe sample shown in FIG. 3B was used as the film to be polished. As canbe seen in FIG. 13, the region on the graph, which indicates thephenomenon that the polishing did not substantially proceed after thesurface was smoothed (to be called a specific region hereinafter) wasshifted to the high content side as compared to the case where ceriumoxide grains having an average diameter of 0.6 μm were used.

[0068] Next, the relationship between the content of the polycarboxylicammonium salt in the polishing solution and the polishing rate alongwith a variable polishing pressure was examined, and the results thereofare shown in FIG. 14. The polishing solution, which was obtained bydispersing cerium oxide grains having an average grain diameter of 0.6μm into pure water at a ratio of 1.0 weight %, was used. The polishingconditions were the same as the standard conditions, and the sampleshown in FIG. 3B was used as the film to be polished. As can be seen inFIG. 14, the specific region was shifted to the high content side as thepolishing pressure was increased.

[0069] From the above results, it can be concluded that the specificregion varies depending on the diameter of the polishing grains and thepolishing pressure.

[0070] The effect of the above-described example was obtained when themolecular weight of polycarboxylic ammonium salt was 100 or more, ormore preferably, 500 or more. Further, when the molecular weight was3000 or more, the effect was more significantly exhibited.

[0071] Further, the surfaces of flat silicon oxide films were analyzedby SIMS (secondary ion mass spectrometry). In the case of theconventional polishing method, about 10¹⁸ atoms/cm³ of cesium wasdetected, whereas in the case of the present invention method, about10¹⁶ atoms/cm³ of cesium was detected. Therefore, it can be concludedthat the polished surface can be cleaned, and the creation of particles,the lowering of the resistance in pressure, and the like can beprevented, with the polishing method of the present invention.

EXAMPLE 2

[0072] The case where the polishing method of the present invention wasapplied to formation of a buried metal wiring portion, will now bedescribed.

[0073] First, as shown in FIG. 15A, a silicon oxide film 52 was formedas an insulation film on a silicon substrate 51. Then, trenches forwiring, each having a width of 0.4 to 10 μm and a depth of 0.4 μm wereformed in the surface of the silicon oxide 52, and a polycrystalline Alfilm 53 having a thickness of 0.6 μm was formed on the surface by aspattering method. After that, the resultant was subjected to a heattreatment, and the trenches were buried with Al, thus obtaining a sample60.

[0074] Next, the sample 60 was subjected to CMP by use of the polishingapparatus shown in FIG. 4, thus forming a buried metal wiring portion.In the CMP, a polishing solution prepared by dispersing silica grainshaving an average grain diameter of 0.05 μm into pure water at a ratioof 10 weight %. The polishing conditions were the same as the standardconditions of Example 1.

[0075] The polishing was carried out until the cross sectional shapeshown in FIG. 15B was obtained. Then, the polishing was carried outagain under the same conditions except that the viscosity of thepolishing solution was adjusted to 4.0 cP by adding polycarboxylicammonium salt thereto at a ratio of 4 weight %. With this polishing, aburied metal wiring portion having a smooth surface without dishing, asshown in FIG. 15C, was formed.

[0076] In the case where the conventional polishing method was used, thedishing of the Al film 52 occurred in a wide metal wiring portion asshown in FIG. 15F, even if the polishing was stopped at an ideal point.In contrast, with the polishing method of the present invention, thepolishing rate was not reduced in a less stepped or wide metal wiringportion, and the dishing was remarkably suppressed.

[0077] More specifically, the dishing amount of the surface of theburied metal wiring layer polished was examined by a probing type levelmeter with regard to each of the conventional polishing method and thepresent invention method. In the case of the conventional polishingmethod, the dishing amount was 1000 to 1500 Å in wiring having a widthof 10 μm, whereas in the case of the present invention method, thedifference was about 500 Å.

EXAMPLE 3

[0078] The case where the polishing method of the present invention wasapplied to formation of a contact, will now be described.

[0079] First, as shown in FIG. 15A, a silicon oxide film 72 was formedas an insulation film on a silicon substrate 71. Then, trenches forwiring, each having a width of 0.4 μm and a depth of 0.4 μm were formedin the surface of the silicon oxide 72, and a tungsten film 53 having athickness of 0.6 μm was formed on the surface by a CVD method, thusforming a sample 80.

[0080] Next, the sample 80 was subjected to CMP by use of the polishingapparatus shown in FIG. 4, thus forming a contact. In the CMP, thepolishing solution used here was prepared by dispersing alumina grainshaving an average grain diameter of 0.05 μm into pure water at a ratioof 10 weight %, so as to have a viscosity of 3.0 cP and pH of 5. Thepolishing conditions were the same as the standard conditions ofExample 1. With this polishing, a contact having a smooth surface asshown in FIG. 16C was obtained without creating dishing, through thecross sectional shape shown in FIG. 16B, was formed.

[0081] In the case where the conventional polishing method was used, thedishing of the tungsten film 73 occurred as shown in FIG. 16D, even ifthe polishing was stopped at an ideal point. In contrast, with thepolishing method of the present invention, the polishing rate was notreduced in a less irregular film or a film having a large protrudingportion, and the dishing was remarkably suppressed.

[0082] In this Example, a chemical effect takes place during thepolishing of the tungsten film by adding ammonium polysulfonate to thepolishing solution since pH thereof is 3.5. Therefore, scratches on thetungsten film can be erased. Further, in the polishing, a stagnant layeris formed on the silicon oxide film 72, scratches on the silicon oxidefilm 72 can be erased.

[0083] In the meantime, the motor current value of the polishing tablewith respect to a change in the viscosity of the polishing solution isillustrated in FIG. 17. As can be seen in FIG. 17, the motor currentvalue was decreased as the viscosity of the polishing solution wasincreased. This phenomenon indicates that as the viscosity of thepolishing solution increases, the frictional coefficient between thepolishing cloth and a hard metal such as tungsten, decreases.Consequently, with the polishing method of the present invention, thedeterioration of the polishing cloth in the polishing can be prevented.

[0084] The present invention is not limited to the above Examples 1 to3, and can be remodeled into a variety of versions. For example, withuse of Fe₂O₃ grains, SiC grains, SiN grains, ZrO₂ grains or TiO₂ grainsas the polishing grains, an effect similar to the above can be obtained.Or, in order to increase the polishing rate, an alkali such KOH, NaOH orNH₄OH or an acid such as HCl may be added and an effect similar to theabove can be obtained. Further, if the temperature of the polishingsolution is changed within a range of 0 to 90° C., an effect similar tothe above can be obtained.

[0085] Further, the present invention is not limited to the aboveExamples in the following respect.

[0086] In the Examples 1 to 3, the descriptions were provided inconnection with the cases where the polishing films are a silicon oxidefilm, an Al film and a tungsten film; however an effect similar to theabove can be obtained in the case where the polishing film is an Agfilm, Cu film, Si film or Si₃N₄ film.

[0087] The organic compound used in the polishing, having a stronginteraction with the film to be polished, is not limited to those usedin the above Examples, and as long as it can serve as a surfactantbetween the film and the polishing solution, any type may be used.

[0088] The average distance between the surface of the protrudingportion of the film and the surface of the polishing cloth shouldpreferably be the same as or less than the average diameter of thepolishing grains, in terms of the polishing rate, the polishingselection ratio and the like. The content of a polycarboxylic ammoniumsalt should be 4.5 weight % or less, for example.

[0089] The present invention can be applied to the formation of anabsorption pattern (W, Cr, etc.) of a mask for exposure, such as a phaseshift mask or an X-ray mask. Or, as long as the essence of the inventionremains, the invention can be practiced in various different versions.

[0090] An example of the method used in the present invention, formeasuring the average distance between the surface of the depressedportion of the film and the surface of the polishing cloth, is a methodof measuring a hight of a chuck holder by using a laser displacementmeter. An example of the method used in the present invention, formeasuring a frictional coefficient between the film and the polishingsolution and that between the polishing cloth and the polishing solutionis a method of measuring a motor current of a polishing table.

[0091] As described above, in the polishing method of the presentinvention, an organic compound having at least one hydrophilic group anda molecular weight of 100 or more is added to the polishing solution,and therefore the average distance between the main surface of the filmand the main surface of the polishing cloth is set wider than theaverage diameter of the polishing grains, or the frictional coefficientbetween the film and the polishing solution is set lower than thatbetween the polishing cloth and the polishing solution. Therefore, onlya predetermined protruding portion can be polished at high efficiencywithout causing dishing.

[0092] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details, representative agents,and illustrated examples shown and described herein. Accordingly,various modifications may be made without departing from the spirit orscope of the general inventive concept as defined by the appended claimsand their equivalents.

What is claimed is:
 1. A polishing method comprising the steps of:forming a film to be polished, having a depressed portion and aprotruding portion on a surface of a substrate; and polishing said filmto be polished by relatively moving said substrate and a polishingtable, while pressing said substrate having said film to be polished,onto a polishing cloth of said polishing table and supplying a polishingsolution containing polishing grains, between said film to be polishedand said polishing cloth; wherein an average distance between a surfaceof said depressed portion of said film to be polished and a surface saidpolishing cloth during polishing is set larger than an average diameterof said polishing grains.
 2. A method according to claim 1 , wherein oneselected from the group consisting of silicon, quartz, sapphire, Al₂O₃and a compound of elements of the groups III and V of the periodictable, is used as a material of said substrate.
 3. A method according toclaim 1 , wherein a film mainly containing one selected from the groupconsisting of SiO₂, α-Si, poly-Si, SiON, SiOF, BPSG, PSG, SiN, Si₃N₄,Si, Al, W, Ag, Cu, Ti, TiN, Au and Pt, is used as said film to bepolished.
 4. A method according to claim 1 , wherein said polishinggrains contain, as a main content, one selected from the groupconsisting of SiO₂, CeO₂, Al₂O₃, Fe₂O₃, SiC, SiN, ZrO₂ and TiO₂.
 5. Amethod according to claim 1 , wherein the average diameter of saidpolishing grains is 0.01 to 5.0 μm.
 6. A polishing method comprising thesteps of: forming a film to be polished, having a depressed portion anda protruding portion on a surface of a substrate; and polishing saidfilm to be polished by relatively moving said substrate and a polishingtable, while pressing said substrate having said film to be polished,onto a polishing cloth of said polishing table and supplying a polishingsolution containing polishing grains, between said film to be polishedand said polishing cloth; wherein a frictional coefficient between saidfilm to be polished and said polishing solution during polishing islarger than a frictional coefficient between said polishing cloth andsaid polishing solution.
 7. A method according to claim 6 , wherein oneselected from the group consisting of silicon, quartz, sapphire, Al₂O₃and a compound of elements of the groups III and V of the periodictable, is used as a material of said substrate.
 8. A method according toclaim 6 , wherein a film mainly containing one selected from the groupconsisting of SiO₂, α-Si, poly-Si, SiON, SiOF, BPSG, PSG, SiN, Si₃N₄,Si, Al, W, Ag, Cu, Ti, TiN, Au and Pt, is used as said film to bepolished.
 9. A method according to claim 6 , wherein said polishinggrains contain, as a main content, one selected from the groupconsisting of SiO₂, CeO₂, Al₂O₃, Fe₂O₃, SiC, SiN, ZrO₂ and TiO₂.
 10. Amethod according to claim 6 , wherein the average diameter of saidpolishing grains is 0.01 to 5.0 μm.
 11. A polishing method comprisingthe steps of: forming a film to be polished, having a depressed portionand a protruding portion on a surface of a substrate; and polishing saidfilm to be polished by relatively moving said substrate and a polishingtable, while pressing said substrate having said film to be polished,onto a polishing cloth of said polishing table and supplying a polishingsolution containing polishing grains, between said film to be polishedand said polishing cloth; wherein an organic compound having a molecularweight of 100 or more, and containing at least one hydrophilic groupselected from the group consisting of COOM₁ (M represents an atom or afunctional group which can form a salt when substituted with a hydrogenatom of a carboxyl group), SO₃H (sulfo group) and SO₃M₂ (M₂ representsan atom or a functional group which can form a salt when substitutedwith a hydrogen atom of a carboxyl group) is added to said polishingsolution.
 12. A method according to claim 6 , wherein said organiccompound is added to said polishing solution so that a viscosity of saidpolishing solution falls within a range of 2.6 to 4.5 cP.
 13. A methodaccording to claim 6 , wherein said organic compound is polycarboxylicsalt or polysulfonic salt.
 14. A method according to claim 11 , whereinone selected from the group consisting of silicon, quartz, sapphire,Al₂O₃ and a compound of elements of the groups III and V of the periodictable, is used as a material of said substrate.
 15. A method accordingto claim 11 , wherein a film mainly containing one selected from thegroup consisting of SiO₂, α-Si, poly-Si, SiON, SiOF, BPSG, PSG, SiN,Si₃N₄, Si, Al, W, Ag, Cu, Ti, TiN, Au and Pt, is used as said film to bepolished.
 16. A method according to claim 11 , wherein said polishinggrains contain, as a main content, one selected from the groupconsisting of SiO₂, CeO₂, Al₂O₃, Fe₂O₃, SiC, SiN, ZrO₂ and TiO₂.
 17. Amethod according to claim 11 , wherein the average diameter of saidpolishing grains is 0.01 to 5.0 μm.
 18. A polishing method comprisingthe steps of: forming a film to be polished, having a depressed portionand a protruding portion on a surface of a substrate; and polishing saidfilm to be polished by relatively moving said substrate and a polishingtable, while pressing said substrate having said film to be polished,onto a polishing cloth of said polishing table and supplying a polishingsolution containing polishing grains, between said film to be polishedand said polishing cloth; wherein a content of an organic compound insaid polishing solution supplied is varied during polishing.
 19. Amethod according to claim 18 , wherein a content of said organiccompound is varied during polishing by adjusting a supply amount of saidpolishing solution and said organic compound.
 20. A method according toclaim 18 , wherein a content of said polishing solution is varied duringpolishing by switching a supply between at least two polishing solutionshaving different contents from each other.
 21. A method according toclaim 18 , wherein one selected from the group consisting of silicon,quartz, sapphire, Al₂O₃ and a compound of elements of the groups III andV of the periodic table, is used as a material of said substrate.
 22. Amethod according to claim 18 , wherein a film mainly containing oneselected from the group consisting of SiO₂, α-Si, poly-Si, SiON, SiOF,BPSG, PSG, SiN, Si₃N₄, Si, Al, W, Ag, Cu, Ti, TiN, Au and Pt, is used assaid film to be polished.
 23. A method according to claim 18 , whereinsaid polishing grains contain, as a main content, one selected from thegroup consisting of SiO₂, CeO₂, Al₂O₃, Fe₂O₃, SiC, SiN, ZrO₂ and TiO₂.24. A method according to claim 18 , wherein the average diameter ofsaid polishing grains is 0.01 to 5.0 μm.